(a) Field of the Invention
The present invention relates to a rechargeable lithium battery. More particularly, the present invention relates to a rechargeable lithium battery having high capacity, excellent cycle-life, and reliability at a high temperature.
(b) Description of the Related Art
Lithium rechargeable batteries have has recently drawn attention as power sources for small portable electronic devices. They use an organic electrolyte solution and thereby have twice the discharge voltage of a conventional battery using an alkali aqueous solution, and accordingly have high energy density.
For positive active materials of a rechargeable lithium battery, lithium-transition element composite oxides being capable of intercalating lithium such as LiCoO2, LiMn2O4, LiNiO2, LiNi1-xCoxO2 (0<x<1), LiMnO2, and so on have been researched.
As for negative active materials of a rechargeable lithium battery, various carbon-based materials such as artificial graphite, natural graphite, and hard carbon have been used, which can all intercalate and deintercalate lithium ions. Graphite of the carbon-based material increases discharge voltage and energy density for a battery because it has a low discharge potential of −0.2V, compared to lithium. A battery using graphite as a negative active material has a high average discharge potential of 3.6V and excellent energy density. Furthermore, graphite is most comprehensively used among the aforementioned carbon-based materials since graphite guarantees a better cycle life for a battery due to its outstanding reversibility. However, a graphite active material has low density and consequently low capacity in terms of energy density per unit volume when using the graphite as a negative active material. Further, it involves some dangers such as explosion or combustion when a battery is misused or overcharged and the like, because graphite is likely to react with an organic electrolyte at a high discharge voltage.
In order to solve those problems, a great deal of research on an oxide negative electrode has recently been performed. For example, amorphous tin oxide developed by Japan Fuji Film. Co., Ltd. has a high capacity per weight (800 mAh/g). However, it resulted in some critical defects such as a high initial irreversible capacity of up to 50%. Furthermore, a part of the tin oxide tended to be reduced into tin metal during the charge or discharge reaction, which exacerbates its acceptance for use in a battery.
Referring to another oxide negative electrode, a negative active material of LiaMgbVOc (0.05≦3, 0.12≦b≦2, 2≦2c-a-2b≦5) is disclosed in Japanese Patent Publication No. 2002-216753. The characteristics of a lithium secondary battery including Li1.1V0.9O2 were also presented in the 2002 Japanese Battery Conference (Preview No. 3B05).
However, such an oxide negative electrode does not show sufficient battery performance and therefore there has been much research into an oxide negative material.